He will need to be a registered user (you can't separate him from others if he is logging in as anonymous.
It will be possible to do what you want if his username is not in any group allowed access to other spaces AND you allow access for him to this particular space via a group he is in or as an individual user.
Please see this page for details on how to assign permissions: https://confluence.atlassian.com/display/DOC/Assigning+Space+Permissions
Just note – if you're allowing access for confluence-users on other spaces and he is in confluence-users, you will have to change that (either change the group you're using for permissioning or somehow remove him from confluence users).
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